Cystic fibrosis (CF; mucoviscidosis) is the most common genetic disorder in the Caucasian population, affecting 1:2500 live births1. CF is associated with a wide-spread defect in the secretory processes of all secretory epithelia, including abnormalities in airways, gastrointestinal and genitourinary tracts and elevated sweat electrolyte concentrations. The blockage of the airways and pancreatic ducts due to abnormally viscous mucous secretions are responsible for the two most clinically important manifestations of CF, that being chronic pulmonary infection and pancreatic insufficiency.
The above manifestations appear related to abnormal ion transport in the secretory epithelia of the affected organs such as sinuses, lungs, pancreas, liver, and reproductive tract1-10. The relative impermeability of epithelial cell membranes to Cl− ions appears to be the primary defect in CF.
CF is caused by mutations in the cystic fibrosis gene (CFTR) located on the long arm of chromosome 7 at position q31. CFTR encodes a 1480 amino acid polypeptide, called Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), which functions as a chloride channel in epithelial membranes11-14. Besides its function as a chloride channel, CFTR regulates other apical membrane conductance pathways15.
The CFTR protein in healthy individuals is found in the apical membrane of epithelial cells, which lines the airways, gastrointestinal tract, and other exocrine ducts in the body. The CFTR protein is composed of 12 transmembrane domains (TMDs), two cytosolic nucleotide-binding domains (NBDs), and a cytosolic R region that contains multiple sites for cAMP-dependent phosphorylation16,17. Transport of anions through the transmembrane helices is controlled by the NBDs. It is believed that these domains interact with two molecules of ATP to form a dimer and that binding/hydrolysis of ATP molecules control CFTR channel opening18. The CFTR chloride channel is phosphorylated by protein kinase A (PKA). Phosphorylation by PKA has only a minor effect on CFTR ATPase activity19 and apparently does not act primarily by influencing binding or hydrolysis of the ATP ligand20 but does promote the association of the two NBDs21.
While several classes of mutation in CFTR have been identified to date, the most common mutation found in >90% of patients of European ancestry is a deletion of Phenylalanine at position 508 (delF508-CFTR)1,22. The F508 deletion, located in NBD1, alters the folding and prevents the full maturation of the delF508-CFTR protein, which is therefore degraded very early during biosynthesis. This abnormal folding of the delF08-CFTR mutant protein is thought to be responsible for its improper cellular localization. As delF508-CFTR is a trafficking-impaired mutant that is retained in the ER, its levels at the apical membrane are reduced dramatically, precluding proper Cl− secretion, which leads to CF23-25.
Over the past few years, several small molecules have been identified that attempt to correct the trafficking and functional defects of the delF508-CFTR mutant, such as compounds 3a and 4a (corr-4-a)26-30, carboplatin, sildenafil or its analogues31-32, VRT-325 and VRT-64033-34. Some of these compounds (e.g. VRT(VX)-809 or VX-770) are now in pre-clinical trials.
Current therapies for the treatment of CF are directed toward treatment of the symptoms or effects of the disease and target the secondary effects of the disease; namely, obstructed airways, malnutrition, and chronic bacterial infections in the lungs. These approaches do not address the primary defect of the disease, the mutant CFTR protein, and thereby the reduced chloride channel activity.